Problem: Allam just finished a great meal at a restaurant in Wisconsin. The sales tax in Wisconsin is $5\%$, and it is customary to leave a tip of $15\%$ for the server. The tip amount is calculated on the price of the meal before the tax is applied. (Sales tax is not calculated on tips.) If Allam ordered a $\$15$ meal, how much money will he have to pay with tax and tip? $\$$
Solution: We have to add the ${5\%}$ tax and the ${15\%}$ tip to the basic price of the meal, ${\$15}$. The tax adds ${5\%}$ to the price of the meal, and the tip adds ${15\%}$ so the total amount Allam has to pay is ${20\%}$ more than the price of the meal. Adding ${20\%}$ to an amount is the same as multiplying the amount by ${1.20}$. The price with tax and tip is ${1.2} \times {\$15}=\$18$.